The present invention relates to an acceleration switch and a method of manufacturing the same.
Recently, many cars have been equipped with air bags. Generally, an air bag system comprises an air bag, an inflator and an ECU (electronic control unit). A sudden change in acceleration caused by a car collision is sensed by an acceleration sensor, which is a part of the ECU. An acceleration sensor for sensing a collision employs, for example, a semiconductor type acceleration sensor, in which a strain gauge is formed on a beam supporting a mass. When an applied acceleration is equal to or greater than a set value, the ECU actuates the inflator to inflate the air bag.
A mechanical acceleration switch 51 has been proposed in the prior art, which is shown schematically in FIG. 10. The acceleration switch 51 comprises a switch body 52, and two inertia weights 53, 54 located in a space inside the switch body 52. The first inertia weight 53 is spherical in shape, and has an eccentric shaft 59 positioned eccentrically relative to its center of gravity. The first inertia weight 53 is rotatably supported at its eccentric shaft by the switch body 52. The first inertia weight 53 is connected to the second inertia weight 54 by a beam 55. The second inertia weight 54 is smaller and lighter than the first inertia weight 53. Also, a pair of terminals 56, 57 provided with contacts at their distal ends are arranged in the vicinity of the second inertia weight 54. These terminals 56, 57 are electrically connected to an unillustrated printed circuit board, on which an acceleration sensor, switching elements and the like are mounted.
When the acceleration switch 51 is accelerated, a movable part composed of the first and second inertia weights 53, 54 and the beam 55 is subjected to the force of inertia, which tends to move the movable part relative to the switch body 52. If the acceleration is great enough, the force of inertia turns the heavy first inertia weight 53 as well as the second inertia weight 54 and the beam 55 about the eccentric shaft 59. Then a push portion 58 provided on the beam 55 comes into contact with one of the terminals 56, and the terminal 56 is pushed into contact with the terminal 57. On the other hand, if the acceleration is relatively small, the first inertia weight 53 cannot revolve around the eccentric shaft 59, and the push portion 58 does not contact the terminal 56. Therefore, the terminal 56 does not contact the terminal 57. In this manner, with the acceleration switch 51, the terminals 56, 57 contact each other only when an acceleration exceeding the set value is applied.
Acceleration sensors are being miniaturized, but miniaturization is presently thwarted by the acceleration switch 51 since the acceleration switch 51 is mechanical as shown in FIG. 10. Accordingly, an ECU provided with a mechanical type acceleration switch 51 is comparatively bulky.
Also, if the acceleration switch 51 were made smaller, the sensitivity of the switch would likely decrease because the weights would not be large enough. Accordingly, there is a demand for smaller, more sensitive acceleration switches.
It is one object of the present invention to provide an acceleration switch that is small and highly reliable. It is another object of the present invention to provide a method of reliably and simply manufacturing acceleration switches that are small and reliable.
To attain the objects described above, in the first aspect of the present invention, there is provided an acceleration switch adapted to be switched between the states of ON and OFF depending upon the magnitude of acceleration applied. The acceleration switch comprises a substrate and a semiconductor chip mounted on the substrate. The substrate includes a first contact formed thereon. The semiconductor chip includes a movable part adapted to move between an open position and a closed position depending upon the magnitude of acceleration applied. The movable part has a second contact. When the magnitude of the acceleration applied is less than a predetermined value, the movable part is located in the open position and the second contact is spaced from the first contact. When the magnitude of acceleration applied is equal to or greater than the predetermined value, the movable part is located in the closed position and the second contact comes into contact with the first contact.
In a second aspect of the present invention, there is provided a method of manufacturing an acceleration switch having a movable part adapted to move between states of ON and OFF depending upon the magnitude of acceleration applied. The method comprises the steps of forming an N-type silicon epitaxial growth layer on a silicon chip composed of a P-type single crystal, forming a first high concentration P-type silicon layer having a predetermined configuration in the N-type silicon epitaxial growth layer, forming a second high concentration P-type silicon layer having a predetermined configuration on the first high concentration P-type silicon layer in the N-type silicon epitaxial growth layer, reforming the first and second high concentration P-type silicon layers into porous silicon layers by subjecting them to anodic oxidation, and forming the movable part by removing the porous silicon layers by alkali etching.
In a third aspect of the present invention, an air bag system mounted on a car to protect on occupant of the car from the shock of a collision is provided. The system comprises an air bag, an inflator for inflating the air bag, and an ECU for instructing actuation of the inflator. The ECU comprises an acceleration switch for outputting an ON signal to the ECU when acceleration of the car reaches a predetermined value. The acceleration switch comprises a substrate and a semiconductor chip mounted on the substrate. The substrate has a first contact. The semiconductor chip includes a movable part adapted to move between an open position and a closed position depending upon the magnitude of acceleration applied. The movable part is provided with a second contact. When the magnitude of the acceleration applied is less than the predetermined value, the movable part is located in the open position and the second contact is spaced from the first contact. When the magnitude of the acceleration applied is equal to or greater than the predetermined value, the movable part is located in the closed position and the second contact comes into contact with the first contact.
Features of the present invention thought to be novel will be made apparent particularly in the appended claims. The present invention as well as its object and advantages will be understood from the description of embodiments, which are preferred at present, with reference to the accompanying drawings.